Particulate matter (PM) emissions from turbocharged direct injection spark ignition engines using gasoline and gasoline-ethanol blends are an increasing concern. A key factor that produces this problem is poor mixing from directly injected fuel (fuel that is directly introduced into the engine cylinder as liquid). The particulate emissions problem is increased in turbocharged engines due to increase in the absolute pressure in the cylinder. The concern about particulate emissions relates to both the total mass of the particulates and the number of particulates. Meeting anticipated European requirements for reducing the number of particulates appears to be especially demanding.
Port and direct injection have complimentary advantages. Due to better mixing, reduced wall wetting and improved evaporation of the fuel, both particulate mass/km and number of particulates/km emissions from port injection, where the fuel is introduced in a region outside the cylinders, are typically less than one tenth those from direct injection (when the fuel is introduced as a liquid into the cylinder). On the other hand, direct injection provides better knock resistance due to greater evaporative cooling of in cylinder charge and thus allows operation at higher levels of torque for a given engine displacement and/or compression ratio. Direct injection can also be used to provide better control of fueling and to further increase efficiency by use of stratified operation to enable lean operation at low loads.